The long-range objectives of these stuidies is to gain an understanding of the developmental factors that lead to the orderly, laminar arrangement of neurons within the visual cortex, and the complex set of neuronal connections that determine its function. In our recent work we have provided the first description of fibronectin(FN)-like immunoreactivity in the developing neocortex of the mouse in a distribution that suggests a role in guiding the ingrowth of cortical afferents or in the consolidation of the cortical plate. We have developed a monoclonal antibody, 2Al, which recognizes a developmentally regulated cellsurface antigen (2Al-A) specific to CNS neurons that may be involved in cellsubstrate or cell-cell adhesion. In tissue culture, 2A1-A is prcminent on qrowth cones and at points of neurite-neurite contact and fasciculation. In the present application, we propose a series of in vivo and in vitro studies to answer the following questions: 1. Is FN a component of the substrate traversed by growth cones of early-arriving cortical afferents or in the migratory pathway of cells that form the cortical plate? We will carry out an immno-electron microscopic determination of the relationship between FN-like immunoreactivity, growth cones, and migrating neurons in the developing cortex. 2. Which cellular elements in the developing cortex produce or sequester FN? We will identify cortical cells producing FN in vitro with cell-type specific antibodies, and use in situ hybridization to identify cortical cells producing FN mRNA in vivo. 3. Is the epitope recoqnized by 2Al carried on a ganglioside? Does the location of 2Al-A suggest that it forms a link between cells, between fasciculating axons, or between growth cones and their substrate? We will characterized 2Al-A biochemically, use immuno- EM to lccalize 2A1-A in the embryonic cortex, and use in vitro assays to determine whether neurite outgrowth and fasciculation are altered by Fab fragments of 2A1 and whether advancing neurites adhere to or avoid substrates coated with 2Al-A. 4. Do radial glia and early cortical neurons have common precursor? Does the common precursor persist after neuronal and glial phenotypes appear? What is the fate of the transient population of radial glia? We will use a recombinant retroviral lineage marker to determine the early lineage relationships of neurons and glia in vitro, and cell-type specific antibody markers to determine whether radial glia are transformed into astrocytes.